The long-term goal of this project is to use a well-characterized model system to define processes governing tolerance versus autoimmunity to self antigens. To this end, we have developed transgenic mice expressing the influenza virus hemagglutinin (HA) as a nominal self-antigen, and have been analyzing the extent and basis by which they induce CD4+ T and B cell tolerance to the HA. In the most recent funding period, we showed that mice expressing HA driven by a MHC Class II promoter (HACII mice) and coexpressing HA-specific CD4+ T cell receptors (TCRxHACII mice) spontaneously develop autoimmunity, with inflammatory arthritis as a prominent disease manifestation. We showed that antibody is not required for arthritis development, and that genetic crosses that increased the frequency of CD4+ T cells expressing the transgenic TCR caused arthritis to develop more rapidly. Moreover, the penetrance of arthritis could be modulated by varying the reactivity of the TCR for the HA, since the majority of mice in which the TCR recognizes HA as an agonist peptide (TSIxHACII mice) developed arthritis, whereas mice expressing a TCR with ~100-fold lower reactivity for the HA (TS1(SW)xHACII mice) developed arthritis with substantially lower penetrance. This proposal will use this model system to understand how interactions between autoreactive CD4+ T cells and a self-peptide expressed by systemically distributed antigen presenting cells (APCs) can lead to the development of inflammatory arthritis. In Aim 1 we will examine the molecular and cellular processes governing autoreactive CD4+ T cell development in arthritic TCRxHACII mice. We will define changes in autoreactive CD4+ T cell phenotype that accompany or precede the development of arthritis, and determine how distinct processes (e.g. peripheral expansion, effector cell differentiation) contribute to the ability of autoreactive CD4+ T cells to cause arthritis. In Aim 2 we will examine how expression of HA in different APC subsets contributes to arthritis development in TCRxHACII mice. We will identify phenotypic changes in MHC Class ll+ cells that are associated with arthritis development, and examine how autoreactive CD4+ T cell development is shaped by expression of a self-antigen in differing amounts and/or by different APC types. In Aim 3 we will assess how perturbing CD4+ T cell:APC interaction impacts arthritis development in TCRxHACII mice. We will determine how disrupting CD4+ T cell activation and/or differentiation affects arthritis development, and examine how infections might increase the penetrance of arthritis among genetically susceptible individuals. These studies will provide fundamental insights into the mechanisms of immune repertoire formation and tolerance, will have general applicability to the processes of autoimmunity, and will exploit experimental models with direct relevance to the diagnosis and treatment of human rheumatoid arthritis (RA).